While commercial lithium-ion batteries (LIBs) perform satisfactorily for most home electronics applications, currently available LIB technology does not satisfy some of the more demanding performance goals for Hybrid Electric Vehicles (HEVs), Plug-in Hybrid Electric Vehicles (PHEFs), or Pure Electric Vehicles (EVs). In particular, currently available LIB technology does not meet the 10 to 15 year calendar life requirement set by the Partnership for a New Generation of Vehicles (PNGV). The most extensively used LIB electrolytes have limited thermal and high voltage stability. Thermal and electrochemical degradation of the electrolyte is considered a primary cause of reduced lithium-ion battery performance over time. Many of the performance and safety issues associated with advanced lithium-ion batteries are the direct or indirect result of undesired reactions that occur between the electrolyte and the highly reactive positive or negative electrodes. Such reactions result in reduced cycle life, capacity fade, gassing (which can result in cell venting), impedance growth, and reduced rate capability. Typically, driving the electrodes to greater voltage extremes or exposing the cell to higher temperatures accelerates these undesired reactions and magnifies the associated problems. Under extreme abuse conditions, uncontrolled reaction exotherms may result in thermal runaway and catastrophic disintegration of the cell.
Stabilizing the electrode/electrolyte interface is a key to controlling and minimizing these undesirable reactions and improving the cycle life and voltage and temperature performance limits of Li ion batteries. Electrolyte additives designed to selectively react with, bond to, or self organize at the electrode surface in a way that passivates the interface represents one of the simplest and potentially most cost effective ways of achieving this goal. The effect of common electrolyte solvents and additives, such as ethylene carbonate (EC), vinylene carbonate (VC), 2-fluoroethylene carbonate (FEC), and lithium bisoxalatoborate (LiBOB), on the stability of the negative electrode SEI (solid-electrolyte interface) layer is well documented. Evidence suggests that vinylene carbonate (VC) and lithium bisoxalatoborate (LiBOB), for example, react on the surface of the anode to generate a more stable Solid Electrolyte Interface (SEI). Stabilizing the SEI and inhibiting the detrimental thermal and redox reactions that can cause electrolyte degradation at the electrode interface (both cathode and anode) will lead to extended calendar and cycle life and enhanced thermal stability of LIBs
Typically, lithium bis(trifluoromethanesulfonyl) imide (available as HQ-115 from 3M, St. Paul, Minn.) is used as an electrolyte additive in commercial lithium ion electrochemical cells to enhance performance Lithium bis(trifluoromethanesulfonyl) imide improves cycle life in Graphite/LiCoO2 cells at high temperature. Similar results are identified in Graphite/Li mixed metal oxide cells. Cycle life improvements achieved by adding lithium bis(trifluoromethanesulfonyl) imide correlates with reduced cell impedance. Lithium bis(trifluoromethanesulfonyl) imide also reduces gassing at the negative electrode and can prevent shorting under high temperature float test conditions with single layer polyethylene separator. Thus cell life and safety are improved using lithium bis(trifluoromethanesulfonyl) imide as an additive in standard electrolytes for lithium-ion electrochemical cells.